1. Field of the Invention
The present invention generally relates to waveguide type optical devices used for optical communication and manufacturing methods of the waveguide type optical devices. More specifically, the present invention relates to a waveguide type optical device where a functional thin film is inserted in a groove forming part formed so as to cross a waveguide layer formed in a substrate and a manufacturing method of the waveguide type optical device.
2. Description of the Related Art
Generally, electro-optical effect type optical control elements used in optical waveguides having a structure where metal such as titanium (Ti) or the like is diffused in a crystal of lithium niobate (LiNbO3) have been known as waveguide optical devices. While this waveguide type optical device has an extremely high response speed, change of refractive index differs depending on polarization directions of lights even if the same voltage or electrical field is applied to the wave guide. Hence, the operation of the waveguide type optical device depends on the polarization directions of lights. As a waveguide type optical device solving such polarization dependence problem, Japanese Laid-Open Patent Application Publication No. 2000-28979 discloses a structure where a functional thin film (thin film type wavelength plate) can be inserted in the waveguide.
FIG. 1 is a perspective view of a related art waveguide type optical device 1A. As shown in FIG. 1, the waveguide type optical device 1A has a structure where a waveguide 3 is formed by diffusion of titanium (Ti) in a substrate 2 made of lithium niobate (LiNbO3) and a thin film insertion groove forming part 4 is formed in the substrate 2 so as to cut the a waveguide 3. A functional thin film 5 such as a thin film type wavelength plate is inserted in the thin film insertion groove forming part 4. Under this structure, it is possible to make polarization non-independency of the waveguide type optical device 1A.
On the other hand, as discussed above, in the waveguide type optical device 1A where the thin film insertion groove forming part 4 is formed in the substrate 2, chipping may happen in the substrate 2 including the waveguide 3 when the thin film insertion groove forming part 4 is formed so that a propagation characteristic of light may be degraded. Because of this, as disclosed in Japanese Laid-Open Patent Application Publication No. 8-313758, in order to prevent chipping in the substrate 2 when the thin film insertion groove forming part 4 is formed, a structure where a process supplemental plate called a fixture is fixed to an upper part of the substrate by adhesive has been suggested.
FIG. 2 is a cross-sectional and enlarged view of the vicinity of the thin film insertion groove forming part 4. More specifically, FIG. 2 shows a waveguide type optical device 1B having a process supplemental plate 6. As shown in FIG. 2, the process supplemental plate 6 is fixed to the substrate 2 including a position where the waveguide 3 is formed by using an adhesive 7. After the process supplemental plate 6 is fixed, the thin film insertion groove forming part 4 is formed by cutting through the process supplemental plate 6 so as to reach the substrate 2, so that chipping is prevented from being generated in the waveguide 3. Thus, the propagation characteristic of light is not degraded.
However, in the waveguide type optical device 1B shown in FIG. 2 having a structure where the process supplemental plate 6 is adhered to the substrate 2 including the waveguide 3 by using the adhesive 7, a layer of the adhesive 7 is formed between the substrate 2 and the process supplemental plate 6. In a case where the adhesive 7 exists in the waveguide type optical device 1B, light propagated in the waveguide type optical device 1B is influenced by the refractive index of the adhesive 7.
A characteristic indicated by an arrow A in FIG. 2 is an intensity (mode) of the light propagated in the waveguide type optical device 1B. Since the refractive index of the adhesive 7 is lower than refractive indexes of the substrate 2 and the waveguide 3, as shown in FIG. 2, the light is locked up at the side of the substrate 2. If such a light enters the thin film insertion groove forming part 4, the light spreads when reaching from the waveguide 3 to the functional thin film 5, so that coupling loss of light when the light reenters the waveguide 3 after passing through the functional thin film 5 becomes large.
In order to solve this problem, the adhesive layer 7 may be made extremely thin. However, the thickness of the adhesive 7 influences the mechanical strength for fixing the process supplemental plate 6 to the substrate 2. If the adhesive 7 is thin, the process supplemental plate 6 may be delaminated from the substrate 2 when the thin film insertion groove forming part is formed and therefore the process supplemental plate 6 may not work as a supplemental plate.